A short time ago, Facebook took over the trademark application for "Face" from a company who operated a site called Faceparty.com. This particular application was opposed by Aaron Greenspan—the fellow who took Facebook CEO Mark Zuckerberg to court over the creation of Facebook—but now has been granted a Notice of Allowance by the USPTO anyway.

The Notice of Allowance means that as long as Facebook pays an issue fee to the USPTO within the next three months, it can enjoy a trademark on the word "face" when it comes to "[t]elecommunication services, namely, providing online chat rooms and electronic bulletin boards for transmission of messages among computer users in the field of general interest and concerning social and entertainment subject matter, none primarily featuring or relating to motoring or to cars"

This basically means that it would be very unwise for you to name your hot new social networking site something along the lines of FaceMeet, SlapToTheFace, or FaceNovel. You don't have too much to fear when it comes to naming products or services outside of what the trademark covers though.